An easy open end is a lid which can easily be opened by a finger without using a jig such as can opener. The material of easy open end mainly adopts aluminum. Aluminum-made easy open end fully occupies the market of beverage cans and food cans. The quantity of aluminum-made easy open end has further increased accompanied with the increasing tendency of adoption of easy open end. The phenomenon suggests that the market does not evaluate the merit of switching the aluminum-made easy open end to the one made by laminated steel sheet.
The steel-made easy open end can be manufactured in a similar manner with the aluminum-made one. That is, a base metal sheet is coated by a resin film for food protection, rust prevention, and other purposes, followed by forming a groove or a carved line, (hereinafter referred to also as score), using a press to allow a finger to easily tear-off the score portion. In the pressing step, however, the resin film is broken to exposemetal in a portion of the lid. Rust is generated at the exposed section with lapse of time for both aluminum-made lid and steel-made lid. On the steel-made lid, however, rust becomes further noticeable, and the product value of the steel-made lid further deteriorates than that of the aluminum-made lid. Accordingly, the steel-made easy open end is subjected to repair by lacquering after the pressing step. The lacquered steel sheet itself is inexpensive as the base material. Since, however, a repair lacquering step is required after score-forming on the lid, economic load becomes heavy. Owing to these reasons, steel sheets are not positively used.
With the background, various trials for eliminating the repair lacquering on the steel-made easy open end have been conducted through the investigations of method for forming score on lid and through the adoption of laminated steel sheet responding to the forming method.
Unexamined Japanese Patent Publication No. 06-115546 tries to eliminate repair lacquering by using a polyester resin and by improving the conventional method for forming V-shape score. Unexamined Japanese Patent Publication No. 09-234534 improves the score-forming method while specifying the thickness and the breaking elongation or resin layer. Unexamined Japanese Patent Publication No. 11-91775 tries to eliminate repair lacquering using a curved surface die for score forming.
However, even in those repair-free technologies for steel-made easy open end, there are cases of incapable of using steel-made easy open end depending on the required level of the contents filled in the can, the design of can and lid, the method for can manufacturing and lid manufacturing, the retorting condition, and the like. Consequently, these technologies are not at a satisfactory level.
In concrete terms, (1) reduction in the score residual thickness by a certain score-forming method results in likely breaking of film, though the openeability (also called the can openeability) improves, thus the elimination of repair lacquering becomes difficult, (2) blushing phenomenon on film occurs during retorting, which is a problems specific to laminate film, and other problems arise. The term “retorting” referred to herein means the heat treatment of a can with packed contents using high temperature steam or the like to thermally sterilize the contents.
Therefore, the market wants a technology that solves these problems. Once attaining the technology, full scale market introduction of the inexpensive laminated steel sheet becomes possible. Furthermore, the significance of the technology does not stay at the reduction in can cost. The substitution of steel for aluminum gives a significant impact also to industries because the all-steel can is superior in the recycle viewpoint and because the steel itself is a base material giving light environmental load compared with aluminum.
An object of the present invention is to solve the above problems and to provide a steel-made easy open end which achieves both openeability and elimination of repair lacquering even with a thin score residual thickness, and to provide a laminated steel sheet as the material therefor. The prevent invention also provides a steel-made easy open end which has excellent blushing resistance during retorting, adding to both of the above performance, and provides a laminated steel sheet suitable as the material therefor.